Many analytes of clinical interest are taken up by cells or become complexed with one or more other components of the test sample. Accordingly, to obtain an accurate measurement of the amount of analyte present in the sample, it is preferable to treat the sample, and/or conduct the assay under conditions, such that the analyte is released from the cells or other component(s) for detection in the assay.
For example, immunosuppressant drugs such as tacrolimus, everolimus, temsorolimus and cyclosporine are effective for the treatment of organ or tissue rejection following transplant surgery, of graft versus host disease and of autoimmune diseases in humans. During immunosuppressant drug therapy, monitoring the blood concentration levels of the immunosuppressant is an important aspect of clinical care because insufficient drug levels lead to graft (organ or tissue) rejection and excessive levels lead to undesired side effects and toxicities. Blood levels of immunosuppressant are therefore measured so drug dosages can be adjusted to maintain the drug level at the appropriate concentration. Diagnostic assays for determination of immunosuppressant blood levels have thus found wide clinical use.
Initially, the immunosuppressant must be extracted and separated from the other components of the patient sample. The bulk of the immunosuppressant drug in the patient sample is present in a complex with various “carrier” molecules, such as binding proteins. Sirolimus, tacrolimus and cyclosporine are found predominately in the red blood cells of patient specimens and are associated with specific binding proteins, FKBP for sirolimus and tacrolimus, and cyclophilin for cyclosporine. To ensure an accurate measurement of the total drug concentration in the specimen, the drug bound to the binding proteins is preferably liberated prior to quantitation. This has been addressed by using detergents to lyse cells and/or organic solvents to denature the sample proteins.
Following its extraction from the binding proteins, the drug can be measured in a number of different ways, including by immunoassay or chromatography with absorbance or mass spectrophotometric detection. Immunoassays for immunosuppressant drugs are available in a variety of formats, but all use the binding of an antibody or binding protein (e.g. FKBP) to the immunosuppressant drug. A commonly used immunoassay is an assay which involves the binding of a first antibody to the immunosuppressant and the binding of labeled immunosuppressant (e.g. acridinium-sirolimus) to the remaining free antibody binding sites, followed by quantitation by detection of the label.